N-acylamino acids are particularly mild anionic surfactants and are therefore known as interesting ingredients for cosmetic formulations. A broader use however involves a complex manufacturing process and high costs linked thereto.
Amphosurfactants, such as betaines in particular, form an important element of cosmetic formulations and are known for the formation of a thick, soft foam having a high stability. Furthermore, due to their good biodegradability and low toxicity, they are used in large amounts in the cosmetic industry. Amphoteric surfactants are also known for their mildness and as amphoteric surfactants they may form strong synergies with anionic surfactants such as SLS or SLES. For this reason, SLES/betaine surfactant mixtures form the market standard.
The synergies between amphosurfactants and anionic surfactants may be of a different nature and, for example, influence the thickening, the foam behavior or the mildness of a mixture. Typical cosmetic formulations which contain both N-acylamino acids and betaines are also present on the market. These correspond to the present trend to develop sulphate-free and PEG-free formulations.
U.S. Pat. No. 8,263,538 discloses the use of N-acylamino acids in combination with amphoteric/zwitterionic surfactants in order to achieve a very mild formulation.
US20150335555 discloses the composition of a sulphate-free cosmetic formulation consisting of zwitterionic/amphoteric surfactants, N-acylglycinates and non-ionic surfactants.
US20150141466 discloses the use of amino acid surfactants and betaines in combination with N-methyl-N-acylglucamines.
U.S. Pat. No. 4,772,424 discloses a formulation comprising a specific composition of sulphate/sulphonate, betaine and sarcosinate in order to obtain an isotropic mixture.
N-acylamino acids are currently commercially obtained exclusively from acid chlorides and amino acids by the Schotten-Baumann reaction, as disclosed in U.S. Pat. No. 6,703,517 for example. Acid chlorides are prepared by toxic chlorinating agents, generating a waste stream, which cam make the acid chlorides thus obtained relatively expensive precursors. The chlorinating agents used, such as phosphorus chlorides for example, are prepared in energy-intensive and environmentally harmful processes.
In the Schotten-Baumann route, stoichiometric amounts of NaCl are formed, which remain in the product mixture.
Oxidation reactions of amidoalcohols to N-acylamino acids have been disclosed by a TEMPO catalyzed method in U.S. Pat. No. 7,439,388 and by heterogeneous catalysis in WO2008019887. The yields and selectivities of this method and the use of expensive catalysts render this method unattractive.
Furthermore, the direct synthesis of N-acylamino acids from fatty acids and amino acids has been disclosed in EP1672055, U.S. Pat. No. 3,836,551, DE4408957. In this method, by-products such as di-, tri- or tetrapeptides often occur, high temperatures are required, the ratio of fatty acids to amino acids is in some cases unfavorable or the use of solvents is necessary.
The preparation of N-acylamino acids is also possible from fatty acid esters (alkyl esters or polyol esters), such as disclosed in U.S. Pat. No. 5,856,538, WO9507881, DE4408957, U.S. Pat. Nos. 4,380,646, 5,898,084, JP57/058653 and US20050027120. Both starting compounds are incompatible due to their polarities and, when reaction is successful, the product solidifes and cannot be melted without decomposition. In the cases of secondary components separation by distillation, such as in the example of methanol in the case of methyl esters, strong foaming can occur. By-products are formed and, in the case of the use of a solvent, the removal thereof is difficult.
The N-acylation method may be carried out by using a polyol, such as for example glycerol, as reaction medium. This means that the solidified product can be further stirred during the reaction. The glycerol remains in the product mixture after the reaction; see in this case WO2013014264, WO2013014265, WO2013014266, WO2013014267 and WO2013014268. In this method, a solid concentrate mainly consisting of N-acylamino acid and glycerol is obtained. WO2014008103 discloses another method in which an aqueous solution of N-acylamino acid is obtained.